1. Field of the Invention
The present invention relates in general to a method and apparatus for cutting round holes into the walls of pipes under pressure. More particularly, the invention relates to a method and apparatus for using independently operated cutting means to holesaw a round hole into a pipe wall and to retain the resulting coupon.
2. Description of the Related Art
The hot tapping of pipes is a very old, established art. However, the machines which are currently in use for this purpose are only marginally satisfactory for critical work, such as steam lines or high pressure lines. The primary problems are related to operator safety, retention of the pipe coupon cut from the pipe, dealing with high pressure conditions, and the accuracy of the cut. Additionally, the existing machines are not sufficiently capable in their operation for dealing with vibrations, flow forces, or jamming. Major improvements are needed for subsea use, particularly for cases where operations must be conducted robotically below diver depths.
Traditional hot tap machines have used a holesaw with an integral rigidly concentrically mounted twist drill as a pilot. Coupon retention has been with spring wires anchored and recessed in the flutes of the twist drill or with Hydrotech's selectably engagable rotary bar mounted in a shank pocket of the twist drill and operated by a coaxial rod within the drill stem. Both approaches fail to maintain adequate coupon retention and alignment in the presence of high flow forces in the pipe, and the flute-mounted spring wires are highly unreliable.
Traditional hot tap machines mount their motors externally and extend their drive shafts through glands in a pressure containing housing. The friction from the glands is so high for high pressure conditions that often the shaft cannot be rotated. Additionally, the feed rate and cutting speed are fixed, while the extended shaft is laterally unstable and lacks adequate stiffness for avoiding shaft windup, axial and lateral vibrations, and excessive lateral deflections. The lack of stiffness is exacerbated by the interrupted cutting environment for the holesaw.
Due to their manually controlled operation, one of the most critical problems for conventional hot tap machines is the necessity for close operator proximity during cutting. In the event that the hot tap fitting and machine housing which are clamped around the pipe for supporting the cut leak or rotationally shift around the pipe axis, the operator is imperiled. Leakages of flammable fluids or live steam pose particularly severe hazards to personnel adjacent the hot tapping operation.
There is a critical need to address these problems with significant improvements in operator safety, machine stiffness and accuracy, machining rates, coupon retention, high pressure capability, operator feedback and adaptability to varying operational conditions while cutting, and remote operability.
A continuing need exists for a hot tap machine which is suitable for high pressure pipes and can be remotely operated, in order to promote operator safety.
Furthermore there is a need to improve the reliability of coupon retention for the hot tapping holesawing operation and to provide a means of maintaining coupon positional stability during the holesawing operation, particularly when the holesaw is completing its cut into the pipe wall.
Yet another critical need is a hot tap machine that is more rigid and has increased vibratory damping, so the pipe can be cut faster and more accurately. In addition, a hot tap machine which has independent variable speed drives for the center drill and the holesaw is also needed.